Print hammer actuator



1962 E. A. BROWN ET AL 3,049,990

PRINT HAMMER ACTUATOR Filed Dec. 20, 1960 3 Sheets-Sheet 1 INVENTOREDGAR A. BROWN GUNFER H. SCHACHT MMWQ ATTORNEY PRINT SIGNALS Aug. 1962E. A. BROWN ET AL 3,049,990

PRINT HAMMER ACTUATOR Filed Dec. 20, 1960 3 Sheets-Sheet 2 1962 E. A.BROWN ET AL 3,049,990

PRINT HAMMER AcTuAToR Filed Dec. 20, 1960 3 Sheets-Sheet 3 FORCE 0FSPRING 11 54 24 32 as H HOLDING T44 T42 FORCE T43 T41 TIME FIG. 5 FIG. 6

. armature.

United States This invention relates generally to a high speedelectromechanical actuator and in particular to an actuator for a printhammer.

High speed printers of the type using a print hammer to strike a movingtype wheel require extremely rapid hammer action which must beaccurately timed. The overall speed of the printer is limited by thespeed and timing accuracy of the hammer-movement since both of thesecharacteristics affect the legibility and quality of the printing.

These requirements have led to the development of electromechanicalactuators for print hammers. Various forms of such actuators exist inthe prior art. Typically, such devices have been operated bymagnetically attracting an armature to a core, and transferring thismovement to a print hammer. This motion produces the print operation.When the magnet is de-energized, the armature is moved away from thecore by a bias spring to prepare the hammer for another cycle ofoperation.

The relatively strong electromagnet required for this type of operationis a disadvantage since the heat dissipated limits the duty cycle of thedevice. Furthermore, the current drawn by the electromagnet is affectedby heat-induced changes in winding resistance. The obvious consequenceof these current variations is poor reproducibility caused by changes inattractive force which affect the actuating time of the device.

In devices which move an armature toward a core through electromagneticattraction, the initial force is relatively small due to the air gapbetween the core and With only a small attractive force available toovercome static friction the initial movement of the armature tends tobe erratic, resulting in poor reproducibility. Since static friction isa primary cause of erratic operation, it is desirable to have themaximum force available at the beginning of movement so that friction isquickly overcome. A working magnet is therefore a poor approach to theproblem of overcoming static friction since its attractive force is theleast powerful when it is most needed.

We have found that a greatly improved actuator results from a devicewherein the armature is restored to a position abutting the core bymechanical means and held there against the action of a bias spring bythe force of magnetic attraction. The print operation is accomplished byreducing the magnetic flux to the point where it no longer holds thearmature against the action of the bias spring, allowing the springbiased armature to move the print hammer very rapidly. By using themagnet only to hold the armature the size of the magnet and the currentrequirements can be greatly reduced. Elimination of the heat dissipationproblem also helps to maintain a more constant attractive force for theelectromagnet so that the release time is essentially unvarying.

Our preferred embodiment uses a powerful spring to rapidly acceleratethe armature which has a flux path sufficient to permit the spring to becontained by the force of magnetic attraction. This could provide amarginal differential between the residual induction and the dropoutvalue of the device with the chance that the armature might fail torelease properly when the magatent net is de-energized. To prevent suchfailure, our invention uses a bucking coil energized in opposition tothe magnet to decrease the flux to a value below the residual induction.This coil is wound with relatively few turns to present a low inductanceto the driver which permits a rapidly changing high current pulse to beapplied to the coil. The opposing flux of this coil causes a rapidreduction of the flux in the core and armature to the point where thearmature is released for movement by the spring. The use of a buckingcoil, in addition to making the device more reliable, provides anotheradvantage by increasing the speed of response. Since the opposing orbucking coil is not continuously energized but only pulsed, the problemof excessive heat dissipation is eliminated. With the use of a buckingcoil variations of release time due to temperature changes of the deviceare kept to a minimum since the resistance and hence the current in boththe holding and releasing coil will be similarly affected and thereforecompensate for each other.

it is therefore an object of our invention to provide an improved printhammer actuator.

Another object of our invention is to provide an electromagnetic printhammer actuator using a mechanical restore and a no work magnet.

It is another object of our invention to provide an actuator for a printhammer in which release of the hammer is achieved by a reversal of thenet magnetic flux in a no work magnet.

Still another object of our invention is to provide a print hammeractuator for a printer in which the actuating time may be easilyadjusted.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiment of the invention, as illustratedin the accompanying drawings.

FIG. 1 is an isometric View, with parts cut away and portions shown inschematic form, of a drum printer embodying the invention.

FIG. 2 is a sectional view of the drum printer in FIG. 1 showing theprint hammer in the ready position.

FIG. 3 is a sectional view similar to FIG. 2 but showing the printhammer in the print position.

FIG. 4 is a sectional view similar to FIG. 2 but showing the printhammer after printing and before it is restored.

FIG. 5 is a BH curve for the pole face portion of the magnetic core usedin this invention.

FIG. 6 is a graph showing the holding force of the magnet as a functionof time for the single coil device and also the embodiment using anopposing coil.

With reference to FIG. 1, a continuously rotating drum 2 has a pluralityof type faces 3 arranged on the periphery in cylindrical bands. Eachtype face contains a complete alphabet and such numbers and specialcharacters as may be required. The drum is rotated at high speed torepetitively present all characters in each face to the printinglocation. Since each face accommodates one print space, there are asmany tracks as there are spaces in a line.

A document 4 is held in position away from the drum but adjacent theretoby suitable means not shown. After a complete line has been printed,document 4 is advanced to a position for printing of the next line. Suchpositioning and feed means are well known and have not been shown sincethey are not part of our invention.

A plurality of print hammers 5, one for each print position, arepivotally mounted on shaft 6. Some hammers have been omitted from thedrawing for clarity. Magnetic means, comprising cores 7 having windings8 and 9, are adapted to engage armature portion it} of print drum 2 toprevent tearing of the document 4 after printing in each position.Springs 14 are not sufficiently strong to prevent or stop the motion ofthe print hammer 5 but after the kinetic energy of the hammer has beendissipated they'operate to hold the hammers away from the document.

The individual hammers are released in response to print signals fromthe decoder 15. The print signal results from coincidence of a datasignal applied to input 16 from a keyboard or other device and a timingsignal applied to input 17 from transducers 18. The data signals areplaced in a storage location according to the line position at which itis to be printed. Each harm-"oer, has its own location in the storagemeans.

The timing input applied to terminal 17 is derived from coaction oftransducers 13 with timing wheel 19 which is made of magnetic material.Transducers 18 are positioned close to the periphery of timing wheel 19.As the wheel is rotated past transducers 18 the variations in fluxcaused by slots 29 induce signals in transducer 18. The slots may becoded to provide a unique combination of signals for each position orletter of drum 2. The output signal of transducers 18 thereforeindicates the letter in position to be printed at that instant. Thesetiming signals are compared within the decoder with the stored printsignals for each print position. When the timing signal corresponds to asignal in storage, a print signal is produced to fire the proper hammer.r

The print signal may operate to interrupt or reverse current through aholding coil '8 on the core associated with the appropriate print hammeror it may be used to energize a second coil 9 in opposition toholdingcoil 8 and thereby reduce the flux in the core to a minimum value. Inthe embodiment shown the print signal is applied to a bucking coil 9which opposes the flux induced by continuously energized winding 8. Thisreleases the hammer 5 for movement toward the print drum to produce theprint operation. After each revolution of drum 2, all print hammers arerestored by the action of restore cam 21 which may be driven by suitableconnection to drum 2 or by separate means operable in response toselected signals from transducers 18 or decoder 15. Since the flux incore 7 is once again suificient to hold armature against the core, theprint hammer is ready for another print operation. a

In FIG. 2 print hammer 5 is shown in position against the core 7.Winding 8 is energized and winding 9 deenergized thereby providingsufficient flux in the core 7 to hold armature 1% against pole faces 22.As shown in the drawings pole faces 22 have an area which is less thanthe cross sectional area of the body of core 7. This allows the body ofcore 7 to be operated below magnetic saturation while maintaining thepole faces fully saturated for maximum efiiciency. The number of turnsand the current in winding 8 is such that the pole faces 22 areessentially saturated over the full range of operating tolerances. Inother words, sufiicient magnetomotive force is induced into the core forall operating conditions to make sure that the pole faces are always atthe saturated region. Sufficient reserve excitation may be providedwithout materially changing the pole face flux density. Since the fluxdensity in the pole face remains constant, the holding force will alsoremain constant despite variations in current through winding 8.

In FIG. 3 the flux in core 7 has been reduced to the point where it isno longer sufficient to retain armature 19 of print hammer 5. Spring 11then urges the print hammer forward so that face 12 presses ribbon 13against document 4 to transfer a character on drum 2 to the document.This is the print position.

As mentioned previously, this release may be accomplished byinterrupting the current to winding 8, reversing the current in winding8, or in the preferred embodiment by energizing bucking winding 9.Although the response time of our preferred embodiment is in the orderof microseconds, the linear velocity of the type track is also quitehigh, being approximately 300 inches per second. It is thereforenecessary to develop the print signal somewhat in advance of the time itis desired to cause printing to occur. This is easily accomplished bylocating the transducers 18 at the proper position.

The spring 14 has partially restored the print hammer 5 in FIG. 4. Theprint hammer is held away from the document 4 and ribbon 13 to preventtearing by rotation of the drum-when the document is advanced. In otherwords, a static balance of the force exerted by spring 11 and the forceexerted by spring 14 leaves the print hammer 5 in the position of FIG.4.

Cam 21 is rotated to restore the print hammer to the ready position. Theshape of cam 21 is such that armature 10 is moved to a position abuttingpole faces 22. Since coil 8 is energized and coil 9 is de-energized thearmature 10 will be retained against the pole faces 22 by the force ofmagnetic attraction when cam 21 has rotated beyond contact with printhammer 5. The hammer 5 is now ready for another print operation.

The hysteresis loop shown in FIG. 5 represents magnetic conditions atthe pole faces 22. When the holding coil 8 is energized the pole facesof core 7 are in the region of saturation, for example point 23. Whencoil 8 is de-energized the remanent flux decays to point 24 on line 25.Line 25 is displaced from the normal axis 26 because of the magneticfield across the air gap between armature 5 and core 7. The holdingforce provided by the remanent flux must be less than the force ofspring 11 or the armature 10 will not release. Furthermore, the holdingforce provided by the saturated pole faces 22 must be greater than theforce of spring 11 or the armature 11} will not be retained. Since themagnetic attraction on armature 10 is proportional to the flux densityat the pole faces 22 it can be seen that the force exerted by spring 11must lie within the range between points 24 and 28. If reliableoperation is to be obtained it must be certain that the armature willnot be released inadvertently. To meet this requirement the operatingpoint is chosen'so that the force of spring 11 is at least,

but not necessarily more than, sufiicient to cause release of thearmature when winding 8 is de-energized.

This overcomes the possibility that the armature may be inadvertentlyreleased but the chance of the armature failing to release is increased.To guard against this eventuality, a second winding 9 on core 7 isenergized by the print signal in a direction to produce an opposing fluxto that of winding 8. This opposing flux drives the pole faces of core 7in the direction of negative saturation, which is point 29, therebyreducing the remanent flux and also the holding force, toward zero. Thisinsures that the armature will be released, since if the opposing fluxinduced by coil 9 is sufficiently large, the holding force acting onarmature 10 will momentarily approach unnecessary to de-energize thewinding 8 since winding 9 may be supplied with a high current pulsewhich causes the flux density to approach zero, thereby insuring thatthe armature will be released.

Another advantage of winding 9 is the compensation of the effects due totemperature changes, as mentioned previously.

A frequently encountered problem with on the fly printers isregistration. Since the type is moving at high speed any lack of properhammer timing results in displacement of the printed character from theproper position. In this embodiment any error in registration is limitedto vertical displacement since the horizontal position is fixed.Although our device achieves greater accuracy than other systems ofcomparable speed, it is desirable that maximum speed be obtained. Smallvariations from hammer to hammer in response time and operatingcharacteristics become significant factors at high speeds. To permitaccommodation of these variations we provide variable resistors 30 inseries with each of windings 8. While these resistors do not affect theholding force of the pole faces 22, since these are at saturation, theydo affect the time it takes the current in the bucking coil to causerelease. It can be seen that the operating point on the BH curve will befurther to the left for an increasing value of resistor 30 and furtherto the right for a smaller value. While the holding force on armature 10remains essentially the same for all values of resistor 30, the releasetime may be varied over a narrow range.

This is better understood with reference to FIG. 6 and FIG. 5 incombination. Assuming that pole faces 22 become essentially saturatedwhen the flux density reaches the level of line 31. Increased excitationto winding 30 increases the ampere turns but the flux density isincreased only sightly. If resistor 30 is set to provide ampere turnsequal to line 32 of FIG. 5, the flux in the core will be at point 33.Carrying this value across to FIG. 6, which represents holding forceplotted against time, it may be seen that the flux decays along curve 34when winding 8 is disconnected. A vastly faster decay is produced whenwinding 9 is energized with a high current pulse as shown by curve 35.

If resistor 30 is decreased to provide ampere turns equivalent to line36, the flux will be at point 37. Carrying this value to FIG. 6, theflux decay will follow curve 38 for the case where winding 8 is merelyde-energized and curve 39 for the embodiment where winding 9 isenergized with a high current pulse.

Since the spring force causes the armature to be released when the fluxdrops below the value represented by line 49, T represents the releasetime for the curve 38 and T represents the release time for curve 34.The difference between T and T represents the adjustment possible withresistor 30. Similarly T and T represent the release times for fluxdecay curves 39 and 35 respectively. 3

FIG. 6, in addition to showing how the actuating time may be controlledby resistor 30, shows how the release or actuating time and thevariation in this time is substantially reduced by the use of a buckingwinding. The interval between T and T is substantially less than theinterval between T and T This shows the reduced influence of variableson the preferred embodiment of our invention.

The use of resistor 30 permits the operator to make an immediateaccurate adjustment to correct any misalignment that may exist.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. In a print hammer actuating device: a core of magnetic materialhaving first and second windings thereon, a print hammer having anarmature portion and movable between said core and print position,spring means urging said hammer toward said print position, mechanicalmeans for restoring said hammer to said core against the action of saidspring means, means for continuously energizing said first winding toretain said restored hammer in engagement with said core, and means formomentarily energizing said second winding to oppose the flux induced bysaid first winding and reduce the flux density of said core whereby saidhammer is released to be moved toward said print position by the actionof said spring means.

2. In a print hammer actuating device: a core of magnetic materialhaving first and second windings thereon, means for continuouslyenergizing said first winding, a print hammer movable between a readyand a print position, an armature portion of said print hammer abuttingsaid core in said ready position, spring means urging said hammer towardsaid print position, mechanical means for periodically restoring saidhammer to said ready position against the action of said spring meanswhereby said hammer is retained in engagement with said core by theinduced flux of said first winding, and means for momentarily energizingsaid second winding to induce a flux in said core opposing that of saidfirst winding and to reduce the flux density of said core and releasesaid hammer for movement toward said print position by the action ofsaid spring means.

3. .A print hammer actuator comprising: a core of magnetic materialhaving first and second windings thereon, a print hammer having anarmature portion and movable between a first, ready, position whereinsaid armature abuts said core and second, print, position; spring meansurging said hammer toward said print position, mechanical means forrestoring said hammer to said ready position against the action of saidspring means, means for continuously energizing said first winding toretain said restored hammer in the ready position with said armatureabutting said core, and means for momentarily energizing said secondwinding to reduce the flux density of said core whereby said hammer isreleased for movement toward said print position by the action of saidspring means.

4. A print hammer actuator comprising: a core of magnetic materialhaving first and second windings thereon wound in opposing relationship,a print hammer having an armature portion and movable between a readyposition and a print position, means fixedly positioning said core toabut said armature when said hammer is in the ready position, springmeans urging said hammer toward said print position, mechanical meansfor restoring said hammer to the ready position against the action ofsaid spring means, means for continuously energizing said first windingto retain said restored hammer in the ready position with said armatureabutting said core, and means for momentarily energizing said secondwinding to induce an opposing flux and reduce the flux density of saidcore whereby said hammer is released for movement toward said printposition by the action of said spring means.

5. In a print hammer actuating device: a core of magnetic materialhaving a winding thereon, pole face portions of said core, said poleface portions having a restricted area operating to increase the fluxdensity in said pole face over the flux density in the remainder of thecore, a print hammer having an armature portion and movable between afirst position abutting said pole faces and a second, print, position;spring means urging said hammer toward said print position, mechanicalmeans for restoring said ham-mer to said pole faces against the actionof said spring means, means for continuously energizing said firstwinding to retain said restored hammer in engagement with said polefaces, and means for momentarily reducing the flux in said core wherebysaid hammer is released to be moved toward said print position by theaction of said spring means.

6. A device according to claim 11 wherein the means for reducing theflux in said core comprises means for momentarily de-energizing saidwinding.

7. In a print hammer actuating device: a core of magnetic materialhaving a winding thereon, pole face portions of said core, said poleface portions having a restricted area operating to increase the fluxdensity of said pole faces over the flux density in the remainder ofsaid core, a print hammer having an armature portion and movable betweena first position abutting said pole faces and a second, print, position;mechanical means for restoring said hammer to said pole faces againstthe action of said spring means, means for continuously energizing saidfirst winding to saturate said pole faces and retain said restoredhammer in engagement with said pole faces, and means for momentarilyde-energizing said winding to reduce the flux of said pole faces wherebysaid hammer is released to be moved toward said print position by theaction of said spring means.

8. In a print hammer actuating device: a core of. I magnetic materialhaving a winding thereon, pole face portions of said core, said poleface portions having a restricted area operating to increase the fluxdensity of said pole faces over the flux density in the remainder ofsaid core, a print hammer having an armature portion and movable betweena first position abutting said pole faces and a second, print, position;mechanical means for restoring said hammer to said pole faces againstthe action of said spring means, means for continuously energizing saidfirst winding to place said pole faces in the region of magneticsaturation and retain said restored hammer in engagement with said polefaces, means for momentarily de-energizingsaid winding to reduce theflux of said pole faces whereby said hammer is released to be movedtoward saidprint position by the action of said spring means, and meansfor adjusting the energization of said Winding to vary the flux densityof said core While maintaining the pole faces in the region ofsaturation to provide an adjustable release time without aifecting theholding force of said pole faces.

9. In a print hammer actuating device: a core 0 magnetic material havingfirst and second windings thereon, pole face portions of said core, saidpole face portions having a restricted area operating to increase theflux density of said pole faces over the flux density in the remainderof said core, a print hammer having an armature port-ion and movablebetween a first position abutting said pole faces and a second, print,position; mechanical means for restoring said hammer to said pole facesagainst the action of said spring'means, means for continuouslyenergizing said first winding to place said pole faces in the region ofmagnetic saturation and retain said restored hammer in engagement withsaid pole faces, means for momentarilyenergizing said second winding toinduce an opposing flux and reduce the flux density of said pole faceswhereby said hammer is released for movement toward said print positionby the action of said spring means, and means for adjusting theenergization of said first winding to vary the flux density of said corewhile maintaining the pole faces in the region of saturation to providean adjustable release time without aifecting the holding force of saidpole faces.

References Cited in the file of this patent UNITED STATES PATENTS2,010,652, Tauschek Aug. 6, 1935 2,547,457 Ghertman Apr. 3, 19513,001,469 Davis et a1. Sept. 26, 1961

